One feature that Pop-11 shares with many AI language systems, though it is lacking in Fortran, Pascal, C, Cobol and other widely used languages is the "garbage collector", or store manager. This is a very important feature of Pop-11 which enormously simplifies the use of programs that create large numbers of temporary structures then discard them. It is also very complicated because of the need to cope with externally loaded programs using languages like C that do not have garbage collections.
Some aspects of the store management are described in REF SYSTEM. Understanding how to avoid unnecessary garbage collection is one aspect of designing efficient programs. There are many hints on this and similar topics in the file HELP EFFICIENCY.